Thread

xwero posted a programming challenge not to long ago that was a bit of fun for everyone involved - I thought I would post another one here. I’ll post my solution tomorrow.

Given a list (A) with n elements, generate a list (B) with n elements such that the ith element of B is equal to the product of all elements in A except for the ith element.

Test sets:

A = [1, 2, 3]
f(A) = [6, 3, 2]

A = [4, 5, 6]
f(A) = [30, 24, 20]

A = [32, 0, -6]
f(A) = [0, -192, 0]

Hey Michael, shouldn’t the first f(A) be [8, 4, 2] ? (or the A array [1, 2, 3]).

ah yeah - my bad, fat-fingered it. Original post change - correct array is [1, 2, 3]

$originals = array(1, 2, 3);
$products = array();

foreach($originals as $key => $val)
{
 $test = $originals;
 unset($test[$key]);
 $products[$key] = array_product($test);
}

EDIT: AAAH You beat me to it m4rw3r :p

This is a bit shorter

$a = array(4, 5, 6);

$b = array();
while($b[] = array_shift($a))
{
    $f[] = array_product(array_merge($a, array_slice($b, 0, -1)));
}

print_r($f);

Given a list (A) with n elements, generate a list (B) with n elements such that the ith element of B is equal to the product of all elements in A except for the ith element.

for some reason the above is not clicking I read it a few times. What does ith mean I don’t understand the ith element. So where is the solution it’s been a day, and why is ith italicized?

given an array of array(4,5,6)
print_r would return Array ( [0] => 30 [1] => 24 [2] => 20 )

I’m not seeing anything please explain I am seeing something there 5*6=30, 6*4=24 5*4=20. I don’t get it.

Are you saying to times every number against every other number besides the first number against itself together and then store their products into an array?

i feel so dumbfounded by your challenge don’t leave me this way no matter how I tried I could not make sense of it?

Raymond, they’re not arrays.  You have to start at 1, not zero.

You go through the list from 1 to i, and take away element i for every iteration - then multiply all remaining values in the list.

So for:
A = [1, 2, 3]

Step 1 (i=1):
f(A, 1) = prod[2, 3]

2*3 = 6

Step 2(i=2):
f(A, 2) = prod[1, 3]

1*3 = 3

Step 3(i=3):
f(A, 3) = prod[1, 2]

1*2 = 2

So then you get f(A) = [6, 3, 2]

Make more sense now?

m4rw3r had the best solution, in my opinion (plus, it’s very similar to my solution):

$a = array(32, 0, -6);

foreach($a as $k => $v)
{
    $b = $a;
    unset($b[$k]);
    $f[] = array_product($b);
}

print_r($f);

Runs very quickly and the code is simple to understand, you know exactly what it does just by reading it.

...you know exactly what it does just by reading it.

:lol: Michael

m4rw3r ‘s 2nd solution is far superior.

$a = array(4, 5, 6);

while($b[] = array_shift($a))
{
    $f[] = array_product(array_merge($a, array_slice($b, 0, -1)));
}

I suggested the one that Michael posted here.

And the other one was just to see if i could get it any shorter 😛